Certain structural plastics, such as high impact polystyrene (HIPS), acrylonitrile/butadiene/styrene (ABS) resins, poly(vinyl chloride) (PVC) resins, thermoplastic olefins (TPO), and the like, exhibit attractive mechanical properties when extruded, molded, or formed into various articles of manufacture. Such articles include, for example, bathtubs, shower stalls, counters, appliance housings and liners, building materials, doors, windows, siding, decking, railings and shutters, lawn and garden articles, marine articles, swimming pool articles, automotive components, and storage articles.
Although these structural plastics otter attractive property characteristics, the properties of their exposed surfaces can be less than ideal. That is, the surfaces of the structural plastics are degraded by light, they can be easily scratched, and/or they can be eroded by common solvents. Consequently, it has become a practice in the industry to apply another resinous material over one or both sides of the structural plastic to protect the underlying structural material and provide a surface that can withstand abuse associated with the use environment. Such surfacing materials are called “capstocks”.
The capstock generally is much thinner than the structural plastic, typically being about 5 to about 25% of the total thickness of the composite comprising the capstock and structural plastic plies. For example, the thickness of the capstock can be about 0.05 to about 2.5 mm, whereas the thickness of the structural plastic ply can be about 1.0 to about 10 mm.
As a class, acrylic polymers, known for their excellent optical characteristics, resistance to degradation by sunlight, hardness, inertness to water and common chemicals, durability, and toughness, are capstocks of choice for various structural plastics. Such acrylic capstocks are described in U.S. Pat. Nos. 6,852,405. 5,318,737 describes the use of an impact modified acrylic capstock for use over PVC by coextrusion.
Multi-layered polymeric structures are useful to take advantage of the properties of the different polymers. However when one or more capstock layers of a multilayer structure comprise an acrylic polymer, various issues may arise. The adhesion between some layers may not be sufficient. The overall structure may have decreased mechanical strength, especially when some amount of scrap is reworked into the structural plastic layers. Juxtaposed layers may have very different melt viscosities, leading to potential manufacturing difficulties when the multilayer structure is obtained by processes such as co-extrusion.
Biopolymers are finding use as environmentally-friendly alternatives for many common plastics in typical applications, such as in packaging materials and bottling. Biopolymers can be defined either as polymers manufactured from a renewable carbon source, or as polymers that are biodegradable or compostable for example, or both. Most biopolymers suffer from poor physical properties and poor weatherability. Polylactic acid is very brittle, resulting in very low impact properties of finished articles.
It is desired to combine the performance and appearance of an acrylic polymer with the environmental benefits of biopolymers.
U.S. Pat. No. 7,666,946 and US 2012/0142823 describe the modification of biopolymers with up to 15 percent of an acrylic modifier.
JP2000185380A, and US2002160201A, describe thin coated polylactic acid films for packaging applications. The coating can be made of an acrylic resin, however its thickness is less than 5 microns, and it has a low modulus in order to improve the films flexibility.
JP2009066915A describes a multilayer film or sheet formed by laminating a plant-derived resin layer and a rubber reinforcing styrene resin layer. The multilayer structure exhibits low environment load, excellent dimensional stability, and excellent durability under a high-temperature and high-humidity environment. However, it suffers from limited resistance to UV rays, due to the nature of the described layers.
U.S. application Ser. No. 13/390,625, US 2007/0276090, and US 2009/0018237 describe a blend of PLA matrix, acrylic polymer and a core/shell impact modifier having either a hard core layer, or a soft core layer.
It is desired to produce a multilayer structure having acrylic and biopolymers as part of the structure, that overcomes the problems of the structures of the art.
Surprisingly it has been found that a multilayer structure containing acrylic polymers and biopolymers can be produced, having excellent surface properties, such as UV resistance, scratch resistance, chemical resistance, as well as excellent adhesion between juxtaposed layers, excellent impact strength, and ease of manufacture. In one embodiment the adhesion of layers is improved by using an acrylic/biopolymer alloy. In another embodiment the addition of a biopolymer to a layer having an acrylic polymer greatly improves the mechanical strength, reinforcing the mechanical strength of the overall structure, including materials that contain rework scrap. In some cases, the addition of a biopolymer to a layer having an acrylic polymer, modifies the melt viscosity of such polymer, and facilitates the manufacture of the overall structure by melt extrusion.
Further, it also has been found that when internal layers, including the structural plastic substrate, comprise one or more biopolymers, first surface capstock layers comprising an acrylic polymer offer significantly improved UV resistance, abrasion resistance, chemical resistance, and surface appearance.